The present invention relates to a method of effecting random access in a time divided mobile radio system having a primary station and a plurality of secondary stations belonging to the primary station. More specifically, the invention relates to a method of access to these secondary stations when they are in an idle mode, i.e. when the stations listen to messages from the primary station within a given geographical area (cell) and when they wish to establish some form of communication with the primary station. The mobile radio system is a time divided system (TDMA or CDMA) or a combined frequency and time divided system (FDMA/TDMA) with so-called digital control channels, which are utilized by the inventive method.
Random access in a mobile radio system, for instance a mobile telephone system, is known both for analog and digital systems. Random access implies generally that a secondary station (mobile station) transmits to a primary station randomly in time an initial message in accordance with a given protocol, requesting access to the primary station. The primary station then responds, by sending a flag to the secondary station for continued communication between the primary and the secondary stations. The access protocol used in TDMA-systems, so-called time divided or slotted Aloha, is described, for instance, in EP-A-0,321,454.
An access method with time divided Aloha is described, for instance, in the U.S. Pat. No. 5,166,929 (Wing Lo). According to this known method, access is effected by a mobile station by sending to a base station reservation messages which denote which time slot and how many such time slots shall be reserved by the base station for the mobile station concerned, in order to enable the mobile station to complete its access message, so called "Reservation Aloha". This known method can be used particularly in a digital control channel and the access messages can be sent in those free spaces that are created in such a channel when certain control information (SACCH, DVCC) normally used in a traffic channel is not used.